The objective of this study is to determine if azathioprine reduces the alloimmune response in children who receive valved allografts at the time of pediatric heart surgery. Nineteen patients have been enrolled in this study, but six withdrew for various reasons. In the group randomized to receive azathioprine, four patients became inevaluable. One patient died within 24 hours of surgery from low cardiac output; one patient withdrew at the patient's request because of difficulty in obtaining blood by venipuncture; two patients were withdrawn by the responsible surgeon, one because of gram-positive bacteremia that responded to intravenous antibiotics, and one because of Clostridium difficile-positive stools. In the control groups, one patient died four weeks after surgery from low cardiac output, and one patient withdrew because of difficulty in obtaining blood by venipuncture. Of the thirteen patients currently enrolled, or who have completed the study, six were treated with azathioprine and seven were in the control group. To date, there is no significant difference in the level of panel-reactive antibodies between the treated group and the control group at the one-month point or at three months after surgery. Patients treated with azathioprine required dose reductions after three months because of mild liver elevations in transaminases and mild neutropenia. All laboratory values returned to normal after dose reduction. Additional patients will be ascertained for this study until a sufficient number are available to have statistical power enabling the investigators to determine if the therapy decreases the titer of PRA or not.